European Pat. No. EP-B-00 77 932, which corresponds to my co-pending U.S. patent application Ser. No. 438,095, now abandoned, discloses apparatus for joining two sheets of metal which lie in a side-by-side relationship. The apparatus comprises a female die having a central anvil member and support members adjacent that anvil member that extend beyond the anvil working face. The apparatus also includes a male die, which is movable relative to the female die, that will move through the two sheets of metal in a direction toward the anvil member's working face. The support members adjacent the anvil have cutting edges at there distal ends and may flex resiliently outwards with respect to the anvil member to accommodate expansion of metal between the anvil and the male die.
In use, the male and female dies cooperate such that both sheets are cut along congruent incisions, with the male die pressing against the anvil member working surface of the female die. The material is deformed so that sheet metal is laterally extruded to engage beyond the edges of the incisions opposite the sheet metal first pierced by the male die. For this purpose the male die or each of the male dies, if there are a plurality of male dies and female dies, has a cutting edge so that the sheet metal piece first engaged by the male die is properly cut as well as the other sheet metal adjacent the female die.
Joints made by means of this known apparatus, in general, have a strength sufficient for most practical applications. However, in some cases extreme joint strength is required which cannot be reached by using the above method and apparatus.
Another joining method and apparatus to produce a joint wherein the joint remains fluid-tight is disclosed in DE-A-31 06 313. According to the teaching of this publication, two sheet metal pieces are joined in a side-by-side relationship by an apparatus having a male die and a female die but where neither die has cutting edges. The result is that no incisions are produced and neither sheet is cut in any way. Instead, both sheets are merely bulged and deformed, one within the other within the joint site, so that material of both sheet metals protrudes to one side. Thereafter, both protruding material portions are laterally compressed to form the final joint structure. For this purpose the female die may laterally open against spring bias. It should be noted that the deformed portion of the sheet adjacent the female die does not contribute to the strength of the joint; on the contrary, it withstands the lateral deformation of the other sheet metal portion so that the joint strength obtained is limited. Moreover, all interengaging surfaces of the two sheet metals are rounded, which further limits joint strength. These considerations, again refer to joints produced with identical materials and when the joints are of comparable size.
Accordingly, it is the object of the present invention to provide a method and apparatus by which joints of this desired increased strength may be produced. "Increased strength", in this context, means higher strength as compared with the strength of joints made by use of the apparatus referred to above, under circumstances which otherwise are the same (i.e. where thickness and the type of the sheet metals being joined are the same, and there is a similar size joint).